Sunday, September 12, 2010

PIR sensor module

Recently bought a couple of passive infrared (PIR) modules. The seller lists the part number as DC-SS015, although I can't be certain that this indeed is true. Unfortunately the seller could not provide any datasheet. It may in fact be a SS015 since the pin designations for the headers are the same as those on a datasheet for the DC-SS015 (see below).

As far as I can tell there are only two ICs on this module. The SOIC-16 (left of center in the photo) does not have any markings. It could be a BISS001 clone. The (4-pin) SOT-89 (lower right in photo) has ""7533-1" printed on it. I googled and this may be a Holtek 3.3V LDO voltage regulator.

The board measures 33mm x 25mm and is marked "Rev1.1." The fresnel lens is hot glued to the board and for now I have not dared remove it for fear of misaligning it.

The black circular thingie beneath the board is just a 35mm film canister to the prop the board up. Needed to keep it level with the table while taking the picture given the domed fresnel lens on the other side.

Here are screenshots of the pertinent pages from a datasheet I found for the DC-SS015 some time ago. I can no longer find the link for the pdf file by Sure Electronics.

The position on the board and designation of the header pins of the module match those shown on the datasheet. The 3-pin header at the bottom center of the board (as per the photo above) are for power and output. Left pin (silkscreened "SW") is ground, center pin (marked "OUT") is output, right pin is Vcc.

The 3-pin header on the upper left corner has a jumper to select between single trigger and re-triggerable modes. The topmost pin is labeled "H" on the module, while the lowermost pin is labeled "L." With the jumper plugged into the two top pins (H and common) the circuit becomes retriggerable, that is, any motion detected will reset the output high holding period to whatever the user has set it to. With the jumper plugged into the lower two pins (common and L) only the initial motion detected is counted and any further trigger will be ignored until the output-high holding period has elapsed. In other words, in single trigger mode assuming PIR module output is initially low, the time between motion is detected (output goes high) and output goes low is constant.

The output-high holding period (for both trigger modes) is user adjustable and is set via a 1Mohm trimmer resistor. That's the orange part on the upper right corner of the board. I've played around with it and the range is from almost zero to 16.75 seconds. The module arrived with the trimmer set at center, which had a measured time of between 9 and 10 seconds.

I measured the current draw using a Fluke 87V with its min-max feature turned on. Input voltage was measured at 5.18VDC (using an ATX power supply) and load was a 2N7000 MOSFET with its gate directly connected to the PIR output (no limiting resistor). Minimum current recorded was 37µA while maximum was 98µA. As you can read from the datasheet, quiescent current is specified at 50µA.

With the module powered from the same supply as above, output voltage was measured with a Fluke 8842A and for this test module, without any load, low was less than 0.03mV and high = 3.31V.

The module has a long initialization time. After powering up output will go high for several seconds, go low for a shorter period of time, high again for several seconds, and then back low. Thereafter, it will be not triggerable for several more seconds. The period for this set up doesn't seem to be fixed, but the whole thing takes about a minute.

This unit is able to detect human movement to 4 meters, but sensitivity at that distance is fair at best. Seems to perform well enough within two meters. Angular coverage is apparently greater horizontally (as per board orientation in the photos above) than vertically. Difficult to give precise values, but 90 degrees would be a ballpark figure.

I've already used one of the modules in a circuit that automatically switches a compact fluorescent lamp inside a pantry. I'll probably do a blog on that soon.